The invention generally relates to an electrical cable, such as (as an example) a multi-conductor electrical cable of the type used in an oilfield wireline logging operation for purposes of analyzing geologic formations adjacent a wellbore.
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivities than those that contain water. Formations that primarily include sandstone or limestone may contain oil or petroleum gas. Formations that primarily include shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, logging operations are often conducted in the well before its completion for purposes of measuring various characteristics of the geologic formations adjacent to the well to help in determining the location of an oil- and/or petroleum gas-bearing formation, as well as the amount of oil and/or petroleum gas trapped within the formation and the ease of removing the oil and/or petroleum gas from the formation.
Therefore, after a well is drilled, it is common to log certain sections of the well with electrical instruments called logging tools. A wireline instrument is one type of logging tool. The wireline instrument is lowered downhole on a cable called a “wireline cable” for purposes of measuring the properties of geologic formations as the instrument traverses the well. The wireline cable electrically connects the wireline instrument with equipment at the earth's surface, as well as provides structural support to the instrument as it is lowered and raised in the well during the logging operation.
The wireline cable typically contains an infrastructure to communicate power to the wireline instrument and communicate telemetry data from the instrument to a surface logging unit. Because downhole temperatures and pressures may reach, for example, 500° Fahrenheit (F) and sometimes up to 25,000 pounds per square inch (psi), the wireline cable typically is designed to withstand extreme environmental conditions. Because wells are being drilled to deeper depths, the electricity and telemetry requirements of the wireline cable are ever increasing. Thus, in view of these more stringent requirements, the wireline cable designer is presented with challenges related to maintaining or increasing the signal-to-noise ratio (SNR) of the telemetry signals, minimizing telemetry signal attenuation, as well as accommodating the delivery of high power downhole.